The present invention relates to systems for determining the locations of moving trains and for transmitting command information to such trains from control stations.
As greater metropolitan areas grow larger and people are commuting greater distances to job centers, public transportation becomes a more desirable alternative to commuting in a personal vehicle. The traditional response in transportation systems to increased usage has been to increase the amount of infrastructure. However, in light rail transit systems, adding parallel track lanes to existing lines is neither cost effective nor typically possible. Rather, the desirable approach is to increase the capacity over existing infrastructure without sacrificing efficiency or safety.
Traffic demand is increasing rapidly, but reinforcement of railway facilities is proceeding slowly. For a subway system, improving the effectiveness of the transportation system means minimizing the headway intervals between two successive trains on the same line. Inherent in the control process to minimize these intervals is the accurate, real-time position location of all trains under conditions with high ambient interference. To satisfy these resolution requirements, conventional detection and signalling methods require placement of sensors and communication devices at short intervals along the tracks resulting in expensive installation and maintenance costs.
Historically, the safety of railway signalling systems has been ensured by means of fail-safe electromagnetic relays. As traffic densities increased, designers of conventional signalling systems have reluctantly incorporated more capable fail-safe electronic equipment. However, conventional signalling systems are now having difficulty providing train position resolution and rapid control communication response capabilities which satisfy newer headway minimization and regenerative braking functions at an affordable cost.
Conventional signalling systems use various combinations of four different train tracking approaches and three different levels of on-board automation. In three of these tracking approaches, tag readers, dead reckoning, and the Global Positioning System, the train determines its own position and then reports its position to the control station. This method supports semi-automatic control in which each train regulates the interval between it and the preceding train. In the traditional approach employing inductive loops, each train's position is determined directly at the control station. This method supports both manual and automatic control. In the New York subway system, humans operate the train in response to presented movement commands. In the Japanese Railway system, automatic train control systems implement the movement commands.
The railroad and transit communities are interested in wireless methods for tracking trains because interlock and electronic blocking equipment account for 1/3 of the total system costs. Similarly, track circuits, signal cables, and grade crossings account for 1/4 of the total system costs. Thus, wayside signalling devices typically constitute over fifty percent of any system's construction costs.
The Railway Technical Research Institute of Japan has concluded that conventional signalling systems will be unable to safely minimize intervals between trains with variable speeds and dwell times on high density lines. Conventional train location processes essentially perform discrete detection at coarse intervals. However, to reduce headway intervals below two minutes as required on high speed and/or high density lines, continuous detection of train locations will be necessary. Furthermore, train positions must be accurate to within 15 feet to support the necessary responsiveness in this real-time control process.
Accurate train tracking, centralized position location, and automatic train control can all be combined to also support regenerative braking. In regenerative braking, one trains' braking is coordinated with another train's acceleration so that external power consumption is minimized. Regenerative braking will also require a communication function which emphasizes rapid responses over data throughput in this real-time control process.
One known technique for train control employs inductive loops, which provide position accuracy to the loop length but loops are typically no shorter than 100 feet, and usually 1000 feet in length, because these loops must continuously exist for the entire track resulting in high installation and maintenance costs. Inductive loops can be used for data communications from the control station to the train but depend on physical contact between the rails and the wheels, so position determination and communications are easily disrupted by rusty rails, oil contamination, and propulsion motor noise.
Tag systems, typically microwave or infrared based, provide locations when the train is in the immediate vicinity of the transponder. Transponders must be installed at intervals equivalent to the position accuracy required or dead reckoning systems are required to interpolate positions. However, train position information is only available on the trains rather than at the control station. Furthermore, tagging systems do not provide communication between the train and control station.
Dead reckoning systems depend on counting axial rotation to determine position. Wheel slip, wheel race, and wheel wear all contribute significantly to position inaccuracy. Short range errors are estimated by software models and long range accumulated errors can only be removed by recalibration from a companion tagging system. However, train position information is only available on the trains rather than at the control station. Furthermore, dead reckoning systems do not provide communication between the train and control station.
The Global Positioning System (GPS) is a satellite based location system. GPS does provide accurate locations. However, train position information is only available on the trains rather than at the control station. Furthermore, GPS is not available in tunnels and does not provide communications between the train and control station.